Qualitative and morphometric radiographic findings in the distal phalanx and digital soft tissues of sound Thoroughbred racehorses

Lameness examinations and radiography of the distal phalanx and associated soft-tissue structures of the front feet of 103 Thoroughbred racehorses, 4 to 9 years old, were performed to determine normal radiographic appearance and morphometry. Of 103 horses examined, 41 were used in the study that were without clinical signs of foot problems or lameness, had raced at least twice prior to radiography, and had raced at least twice more in the 6 months after radiography. Lateromedial and dorsoproximal-palmarodistal radiographic views of each front distal phalanx were used to measure 28 bone and soft-tissue structures, and to evaluate 14 radiographic findings. Significant differences were not observed between left and right digits for any radiographic determination. Mean thickness of the soft tissues dorsal to the distal phalanx, which provides an evaluation of the epidermal laminae, was 14.6 +/- 1.0 mm when measured adjacent to the distal aspect of the distal phalanx. Most horses had straight, smooth hoof walls that were parallel to the dorsal cortex of the distal phalanx. The mean degree of palmar rotation of the distal phalanx was -0.5 +/- 1.3, and none was rotated more than 4 degrees. The dorsal cortex was smooth and straight, without bone deposition or reaction in either digit for only 5 of the 41 horses. Active bone formation was seen unilaterally along the middle portion of the dorsal cortex in 7 horses, and along the distal portion of the dorsal cortex in 4 of the phalanges from 3 horses. New bone formation along the distal dorsal cortex was often accompanied by resorption of the palmar cortex. For 26 of the 31 horses without active bone deposition, smooth inactive bone formation along the midportion of the dorsal cortex was identified in 1 or both distal phalanges. Bone at the solar margin of the distal phalanx was uniformly dense and finely trabeculated, without evidence of resorption or fractures. Severe irregularity of the solar margin was not found in any digit, and the margin of both phalanges was smooth in 8 horses. Various degrees of solar margin irregularity were observed in the other 33 horses. The mean number of vascular canals within the distal phalanx was 8.4 +/- 1.7, and the diameter of the largest canal was 3.4 +/- 0.6 mm. A mean number of 2.0 +/- 1.2 vascular canals was oriented parallel to the radiographic beam on the dorsoproximal-palmarodistal view, and these were termed end-on vessels, because they were visualized as radiolucent dots greater than or equal to 1 mm in diameter in the central portion of the distal phalanx. Racing performance of horses with subtle radiographic signs of laminitis (palmar rotation, hoof wall curvature or undulations, palmar cortical resorption, distal dorsal cortical bone deposition) was poorer than that of horses without these signs. These findings are suggestive of a subclinical laminitis condition, which may influence performance without causing overt clinical signs.